This invention relates to a spin coating apparatus and method and, more particularly, to an apparatus and method for forming a substantially uniform film on a spinning surface.
Spin coating is a well known method for forming thin films on a surface. For example, spin coating is used in manufacturing semiconductor integrated circuits because one step in semiconductor photolithography processing involves coating thin photoresist films on a semiconductor wafer. Spin coating may also be used in other semiconductor manufacturing steps including forming polyimide and silicon dioxide films. Spin coating is also used for forming films in other applications including magnetic disks, lens coatings, reflectors, liquid crystal displays and screens. Spin coating is well adapted to achieve a film of fairly uniform thickness across a surface.
Conventional spin coating involves depositing a liquid on a surface which is spinning about an axis. A typical spin coating apparatus and method is shown in FIG. 1. Referring to FIG. 1, a spin coater 10 is shown specifically adapted for applications in semiconductor manufacturing. Surface 12, which may be a semiconductor wafer, is placed on a spinning member 14, for example a semiconductor wafer chuck, which spins about an axis perpendicular to surface 12. Spinning member 14 is contained within a partially open chamber 16. A liquid is deposited onto the surface by a nozzle that is either stationary above the surface or that follows a predetermined path above the surface. During the liquid deposition the surface may or may not be spinning. Typically after the deposition is completed, the spin rate is rapidly increased to a final spin speed. The time duration for the spinning will vary depending on the specific desired results. After spinning, only a thin film is left on the surface. The pressure within the chamber 16 and across the surface is substantially atmospheric in view of the top opening to the chamber and the typical role of exhaust.
A spin coating liquid is often composed of a nonvolatile material (i.e., a material with a low evaporation rate) dissolved or dispersed in a volatile medium, (i.e., a material with a higher evaporation rate). After the liquid is deposited, centrifugal force causes much of the liquid to flow off the surface. Simultaneously, the volatile medium evaporates. Due to both the centrifugal force and the evaporation, the liquid is converted to a substantially nonliquid thin film comprising the nonvolatile material. The effects of both the centrifugally driven flow of the liquid and the evaporation of the volatile medium from the liquid determine the thickness profile of the final film.
An exemplary thickness contour uniformity map obtained using a conventional spin coater 10 is shown in FIG. 2. More specifically, FIG. 2 displays the uniformity of a photoresist film on a six inch diameter semiconductor wafer 22. Liquid photoresist is applied to the surface of the wafer and, after spinning, forms the relatively dry and nonvolatile (as compared to the liquid) photoresist film used for photolithography processing. Typical liquid photoresist comprises nonvolatile materials including a polymer resin (such as novolac) and a photo active agent (such as naptho quinone diazide) dissolved or otherwise dispersed within a volatile solvent such as ethyl lactate or one-methoxy-two-propanol acetate. In this dissolved or dispersed form, a photoresist is frequently referred to as a liquid photoresist. Examples of available liquid photoresists include EL-215.5AN available from Dynachem and OCG-895.I available from OCG. The photoresist film in FIG. 2 was formed by spinning a liquid photoresist on a wafer at 2000 rpm under an exhaust flow of 100 lpm. The mean film thickness, indicated by heavy contour line 24, is approximately 16,731 angstroms and each contour interval is approximately 5 angstroms. The substantial nonuniformity of the film shown in FIG. 2 hinders the development of advanced semiconductor manufacturing technologies.
It is known that various process variables affect the centrifugal flow and the evaporation. The spinning speed, spinning time, spin acceleration, dispense quantity, and dispense technique are all known to affect the centrifugal flow and the final film thickness uniformity. In addition, the volatile medium, liquid viscosity, resist temperature and exhaust flow rate are also known to affect the evaporation rate and the final film thickness uniformity.
However, adjusting these variables does not adequately address the nonlaminar and turbulent gas flows that are created across the spinning surface. Generally speaking, nonlaminar and turbulent gas flows cause nonuniform evaporation rates across the spinning surface resulting in thickness nonuniformities across the surface. Nonlaminar and turbulent gas flows generally produce a nonuniform evaporation rate of the volatile medium. Thus, a nonuniform evaporation rate across the surface will result in a nonuniform final film thickness.
In addition to the process variables known to those skilled in the art, various apparatus modifications have been used to optimize spin coating film uniformity. For example in U.S. Pat. No. 5,070,813 to Sakai et al. the exhaust flow rate for a coating apparatus may be changed during the coating process in order allegedly to optimize the coating operation. The exhaust rate is monitored by an exhaust rate detection system. The flow rate detection system operates, in part, on the principle that a gas flow will inherently result in a slight pressure drop. The apparatus is open to the atmosphere so the exhaust flow will inherently cause a slight pressure reduction below atmospheric pressure. However, such slight pressure reductions still result in a nonlaminar and turbulent gas flow across the surface and thus the nonuniform evaporation effects caused by the gas flow above the wafer are not addressed.
U.S. Pat. No. 4,587,139 contemplates the introduction of a high kinematic viscosity gas, such as helium, to promote laminar flow near the surface of a disk substrate on which a magnetic ink is being coated in order to reduce the Reynolds number of the gas flow above the disk. While such an approach will result in the promotion of laminar flow characteristics, helium is relatively expensive and may require a gas recovery system, thus lending unnecessary expense and complexity to the coating apparatus.
In U.S. Pat. No. 4,640,856 to Kuo, the centrifugal force variations across a spinning semiconductor wafer are minimized by placing multiple semiconductor wafers on a large spinning disk. Placement of the wafers at a distance away from the spin axis of the large disk is alleged to increase the centrifugal force uniformity across an entire wafer. However, the nonuniform evaporation effects caused by the gas flow above the wafer are not addressed.
One spin coating apparatus for suppressing turbulent air flow above a spinning surface is described in U.S. Pat. No. 5,069,156 to Suzuki. In this apparatus a spinning wall surrounds the spinning surface in order to modify the air flow above the surface. The spinning wall allegedly slows the air speed relative to the spinning surface by directing the air in the same rotating direction as the surface. The modified air flow is said to reduce the aerodynamic forces of the air on the liquid and thus improve the final film thickness uniformity. However, this apparatus still results in turbulent air flow and does not address thickness nonuniformities which result from nonuniform evaporation rates caused by turbulent air flow.
As shown above, a difficulty with conventional spin coaters is their practical inability to substantially reduce or eliminate turbulent air flow patterns associated with film nonuniformity. Instead of focusing upon the elimination of turbulent gas flows, conventional coaters deal primarily with the process variables which affect the centrifugal flow and evaporation such as, e.g., spinning time, spin acceleration, dispensed liquid quantity, etc. While these process variables are important in maintaining uniformity, proper monitoring of the air flow pattern above the spinning surface is equally important.